1. Field of the Invention
This invention relates to single lens reflex cameras with a movable mirror having a half-mirrored portion.
2. Description of the Prior Art
In general, the single lens reflex cameras of this kind have an optical system as shown in FIG. 1, where light passing through and emerging from a photographic lens 1 is reflected by a movable mirror 2 to a focusing screen 3 on which an object image is formed. This image is observed through a penta prism 4 and an eyepiece by the photographer. The half-mirrored portion of the movable mirror 2 passes a portion of the light from the photographic lens 1 to an auxiliary mirror 6 from which the light is reflected to a photosensitive element 7 constituting part of a light meter or a focus detector.
If such movable mirror of the single lens reflex camera is made half-mirrored over the entire area, it results in that, as shown in FIG. 2(a), besides the ordinary rays 10 reflected from the front surface of the half mirror, there are rays 12 passing through the thickness of the half mirror and reflected from the back surface thereof. Therefore, when a luminous object such as a fluorescent lamp lies in the target area, the field of view of the finder presents a ghost 9 in addition to the right object image 8 as shown in FIG. 2(b). Another disadvantage arising from making the entire area half-mirrored is that the viewfield of the finder becomes dark.
Also, in the past, as shown in FIG. 3(a), only an effective portion 14 of the movable mirror was made half-mirrored while the other portion 13 than that was made perfectly reflective, thereby the brightness of the finder image was improved, and the ghost from the back surface of the mirror was reduced. Since this mirror has, however, an abrupt change of reflectivity at the boundary between the central half-mirrored area and the outside total reflective area, a partial darkening 15 appers in the field of view of the finder as shown in FIG. 3(b). As the size of aperture opening of the diaphragm decreases, this becomes more prominent, causing the finder image to look unpleasant.
To improve the partial darkening of the viewfield along with the brightness of the finder image and the ghost, the movable mirror may be constructed in such a way as shown in FIG. 4(a). This is to make it a really half-mirrored so that the area ratio of permeability at the central portion is highest, and the ratio of the total reflection areas gradually increases with increase in the distance from the center to the margin. With the use of such a mirror, because the fine pattern lies in the central portion of highest frequency of use in the finder, flare is produced by diffraction in the image formed in the center of the viewfield of the finder. Particularly the center of the viewfield of the finder is used for focusing adjustment. As the image is associated with flare, therefore, the visual focusing is difficult to perform. Another problem is that when the diaphragm is closed down or when the effective F-number is slow as in closeup photography, the pattern that was not conspicuous gives birth to a hazy image of itself as illustrated in FIG. 4(b), rendering the object image superimposed on that image unpleasant to look, and also making it substantially impossible to perform accurate focusing.
To solve these problems, such a movable mirror as shown in FIG. 5(a) has recently been proposed. This movable mirror 2 has a boundary area of which reflectance gradually increases with increase in the distance from the central or half-mirrored area 14 to the total reflection area 13. The production technique for such movable mirror is that when a reflective coating is applied to a glass plate, a vacuum evaporation mask 17 is set up slightly away from the surface of a glass substrate 2 as shown in FIG. 5(c). It is said that by this method the thickness of the perfectly reflective coating applied to the half reflective coating on the glass substrate can be gradually increased with increase in the reflectance. Upon making experiments according to this method, however, the following defects have been found out:
(1) The adjustment of the reflectance to a gradual variation is very difficult to control. That is, though it does not matter so much when the mask 17 is accurately aligned to an evaporation source 18 as shown in FIG. 5(c), in case when the accurate alignment is not assured as in the mass production, the gradient of reflectance changes from mirror to mirror.
(2) With the gradually varying thicknesses of the total reflection coating in the boundary area as shown in FIG. 5(d), as the thinner part becomes equivalent to one layer of the multi-layer coating, the color characteristics of the half mirror at that part is caused to change. As a result, the boundary between the total reflection area and the half-mirrored area is colored. In the field of view of the finder, therefore, a colored ring 16 appears as shown in FIG. 5(b), giving unpleasant impression to the observer.
It is therefore an object of the present invention to eliminate the above-described drawbacks and to provide a single lens reflex camera with a focusing screen made to have a high diffusing property, a movable mirror made constructed in such a way that the boundary area between the total reflection area and the half-mirrored area is finely divided into parts, some of which are perfectly reflective and the others of which are half reflective, in gradually varying ratios of the sizes of the areas, and mask positioned on the back surface of said movable mirror for shutting off the light passed through the boundary area of the finely divided pattern, thereby giving an advantage that despite the use of the half-mirrored area, the finder functions as if the movable mirror is perfectly reflective over the entire area thereof.
Other objects of the present invention will become apparent from the following detailed description of embodiments thereof with reference to the accompanying drawings.